The AR seems to be the go-to rifle right now if one chooses a rifle for self defense, such as in a home defense situation.
With the shift to the left we have taken with the loss of the House, Senate, and Presidency, it would seem possible and likely probable that we will see gun restrictions passed. These are likely to target semiautomatic rifles like the AR, Mini-14, AK, and many others.
Time will tell I suppose but I think you're seriously misreading the state of things. Dems still hold the House but they significantly lost ground (I think a net loss of 12 seats IIRC). The Senate went from a very slim Repub majority to a dead tie (with the tiebreaker going Dem). And the Dem president is only there because of a historically weak incumbent, and by any objective measure he's center-right. Yeah, he's perhaps more anti-gun that the previous POTUS although we lost more ground in the last four years than in the previous eight. Still there are five-right wing extremists on SCOTUS so sweeping new gun bans seem unlikely. Again, we can return to this in a few years and see who was right!
I didn't quote the second part as I figure you accidentally mistyped as semi-auto rifles are already widely available. So I'm working on the assumption that you meant "unavailable".
So we're clear I'm a gun nut! This is a gun forum so you'd expect that, right? So now that we're clear I have to say that probably a pretty small minority of homeowners/occupants rely on an AR for HD to begin with. The AR is arguable the most popular rifle in America, at least going by the oft-repeated claims in the gun rags and online forums. It's hard to say if that's true at all. My own experiences and gut feeling is that a
simple majority of homes do not have a dedicated HD weapon set up at all. Of those who do have firearms set up expressly for that reason, pistols are probably the most common choice. I could be wrong but from the large commercial YouTube channels, online gun mags and discussions with my gun community friends IRL leads me to that conclusion. One reason for this is the persistent belief that a 5.56 bullet will overpenetrate more than a handgun; this myth is persistent and no amount of "box of truth" videos seems to change this belief. Plus handguns are smaller, cheaper and much more common than ARs.
But if somehow ARs and other semi-auto rifles were banned, would there be other good options? Yes and no! To some extent the rifle is only better option because of the unique virtues of the AR platform. As great as a rifle can be for HD it does pose some disadvantages. It's six times longer and four times heavier than a sidearm and requires both hands to use. The sound level is deafening compared to a sidearm, too. ARs are generally a bit shorter and lighter than some of the other long guns in the same general class (eg CZ Bren, FN SCAR, AK-47, etc). The modularity of the AR, the versatility and the light weight compared to other rifles coupled with the 30 round capacity are probably the main reasons that rifles are even as popular as they are for HD. Substitute another light rifle with a 10 round capacity and you saw one leg off the stool, and now it's pretty wobbly.
If I couldn't use a semi-auto rifle for HD I probably wouldn't go with a lever or bolt gun, I'd go with a shotgun. In fact, I already do keep my Mossberg 500 set up as the backup. My situation isn't universal but for me a shotgun would probably be better than a lever or pump-action rifle. Again, this is an issue specific to me but I'm not particularly good with a lever gun due to not really using them much growing up. If semi-autos were banned I suppose I might pick up a .44 Mag lever and start training with it, and if so then it would move up the list a bit. But if whatever ban was specific to magazine fed rifles and not to shotguns, I'd probably switch to a Beretta 1301 for HD with a sidearm to back it up.
As one of the previous posters mentioned, lever guns can be a good option but they can get tied up if they're not operated properly. This applies to a pump gun as well (although in that case a short-shuck is easier to remedy). If some hypothetical band were to become a reality I'd probably pick up a .44 Mag lever and work on skills. Probably the firearms industry would pivot to skirting restrictions much as they do now, and I could see pump action rifles being developed which could be good alternatives. Mechanically a pump would require a little less travel in the action to cycle a pistol cartridge vs a 3" shot shell although pump rifles would be about the same. I can't recall the manufacturer but I once saw a pump action AR, so the thinking is already out there.
To me a pump action rifle in .44 Mag would be pretty interesting for HD and for camping/hiking. Since I don't have a crystal ball and have no idea what "features" may be banned it's impossible to design a firearm around them right now.